As the degree of integration of semiconductor devices increases, the size of the contact holes becomes increasingly smaller. Such contact holes in these highly integrated devices inevitably have a high aspect ratio, that is, a smaller layer opening area compared with the depth of the hole through the layer. Accordingly, it is important that contact hole filling materials, used for example in an ohmic contact conductive layer/metal barrier layer structure, have good step coverage. Therefore, there is a continuing need for improved processing steps for depositing a thin film and for controlling the thickness of the thin film by controlling the deposition rate, thereby repeatedly depositing the thin film within a tolerance measured by .ANG. units.
Generally, Ti (titanium)/TiN (titanium nitride) layers or Ti (titanium)/WN (tungsten nitride) layers are widely used for the ohmic contact conductive layer/metal barrier layer. The Ti/TiN layer (or Ti/WN layer) is formed by a sputtering method, but the sputtered Ti/TiN layer has poor step coverage and it is also difficult to control the thickness thereof within atomic layer unit tolerances. Although the CVD (chemical vapor deposition) method exhibits good step coverage compared with the sputtering method, it is disadvantageous due to its high impurity content such as fluoride F (in case of WN) or chloride Cl (in case of TiN).
In an effort to solve the impurity problem, some have utilized methods involving increasing the deposition temperature to about 650.degree. C. or more, or to employ a PE-CVD (plasma enhanced chemical vapor deposition) method. See, e.g., Steven D. Marcus et al., "Characterization of Low Pressure Chemically Vapor-Deposited Tungsten Nitride Films", Thin Solid Films 236, pp. 330 to 333 (1993). In Marcus et al., the WN film is formed at a temperature of about 650.degree. C. or more so as to suppress the high impurity content, i.e., fluoride (F). However, the method has some drawbacks, for example, a high thermal budget, difficulty in maintaining the apparatus, and particle content.
On the other hand, the PE-CVD (plasma enhanced chemical vapor deposition) method exhibits poor step coverage compared with the thermal CVD method.
U.S. Pat. No. 4,058,430 discloses an ALD (atomic layer deposition) method. Like the CVD method, the ALD method relies on a chemical reaction between various precursor gases. However, unlike the CVD method, the gases in the ALD method are not mixed in a chamber, but rather the gases are introduced into the chamber one after another in pulses. In other words, as distinct from the CVD method, in the ALD method the precursors are introduced on the substrate alternately.
For example, suppose that a layer C is formed by using gases A and B in the ALD method. First, only the gas A is introduced into the chamber and then the gas A is chemisorbed into a semiconductor substrate. After that, the gas B is introduced into the chamber and then chemisorbed into the semiconductor substrate, thereby forming the layer C. For this reason, regardless of the surface morphology, the ALD layer always has an excellent step coverage, i.e., 100%.
M. Ritala et al., "Atomic Layer Epitaxy Growth of TiN Thin Films", J. Electrochem. Soc. Vol. 142 No.8, pp.2731 to 2737 (1995), employs an ALD method and states that a TiN layer can be formed at a lower temperature of 500.degree. C., compared with 650.degree. C. or more in the CVD method, and with a low impurity content. However, Ritala et. al. has a low deposition rate of 0.2 .ANG./Cyc when a TiN layer is deposited by the ALD method.
Accordingly, there exists a need to form the Ti layer (ohmic contact layer) by a suitable method other than the ALD method, for example a sputtering method, and then subsequently form an ALD-TiN layer as a metal barrier layer without causing defects in the ohmic contact layer, while increasing the deposition rate of the TiN layer.